ListCallback to complement SideEffect and DataFrame callbacks

[mmuurr]

Related to (PR) #520.

There are times where it's useful for the chunked callback to return a list (that is combined into a single list at the end).
An example is when processing a large JSONL file with purrr functions: (1) read file chunk, (2) use purrr to transform, filter, reduce, etc. as needed, and (3) combine into a final list, call that final object my_data.
My specific use-case is to filter the data to discard irrelevant data such that the final object can fit into memory for interactive work/exploration.

When downstream functions expect my_data to be a data frame, then DataFrameCallback works just fine.

But if there already exist some downstream functions written to deal with my_data as a list (e.g. prepared purrr-enabled pipelines), the DataFrameCallback must be written to return a single list-column data frame, then extract that single list-column, and finally pass the now-extracted list to the downstream functions.

It seems silly to return a single-column data frame, just to then extract that single column later, when we know we'd like have a list in-hand at the end of the chunked reading/processing phase.

Minor suggestion, but certainly not a blocker/bug/etc. (since the work-around is described above).
In all other aspects, readr's great!

[hadley]

This seems like a good idea

[jimhester]

Closed by #601